This functionality is new as of version 4.0.
While the Purchase Order is in the 'New' Status, the Purchase Order can be deleted.
If the Purchase Order is not in the 'New' Status, Purchase Order Items that were incorrectly entered, or otherwise not received, can be Cancelled.
Refer to Cancel Ordered Quantity.
To Delete a Purchase Order:
On the 'Purchase Order Details' screen, for a 'New' Purchase Order.
Click the ‘Delete PO’ button.
Confirm the action.